1. Field of the Invention
The present invention relates to the field of pressure relief valve systems, and more particularly, to pressure relief valves for preventing excess fluid pressures in well production tubing due to a line blockage or other problem.
2. Description of the Related Art
The following terms are used in this document: biasing force is the spring force applied to the seating element of a relief valve that keeps the valve in a normally closed position. Cracking pressure is the pressure of a fluid that is sufficient to overcome the biasing force of a normally closed valve, thereby causing the valve to open. A production string is a series of connected lengths of tubing or pipe that extend from the producing zone of a well to the ground surface.
There are several examples of valves used for various purposes in oil, gas and water wells that are the subject of issued patents or pending applications, but none of these inventions includes the novel features of the present invention. The present invention is a pressure relief valve that is positioned in a production string for the purpose of bleeding off excessive fluid pressure from the inside of the production tubing in the event that the fluid pressure in the tubing rises above a preset safe level, thereby preventing tubing ruptures or other overpressure-related failures in the production system. The present invention incorporates several novel features, including a valve element positioned at the center of a biasing leaf spring with the leaf spring fixed at each end, and a thick-walled housing that provides maximum strength to resist tensile and compressive forces placed upon the valve when it is installed in a production string.
U.S. Pat. No. 8,561,703 (Mahmoud et. al., 2013) and U.S. Patent Application No. 2014/0034326 (Mahmoud et al) describe a gas-lift valve that is designed to allow a controlled volume of gas to enter the production string via the annular space between the production string and the well casing, for the purpose of reducing the density of the produced fluid and thereby increasing the production flow rate. The key feature of this invention is a valve assembly comprising multiple sealing surfaces, wherein at least one of the sealing surfaces is optimized to provide a seal under low pressure conditions, and at least one of the other sealing surfaces is optimized to provide a seal under high pressure conditions, thereby enabling the valve to provide a seal over a wide pressure range. This invention utilizes a coil spring to provide biasing force on the valve element.
U.S. Pat. No. 5,564,502 (Crow et al., 1996) describes a control valve designed to selectively allow or shut off flow within a production string. The device is typically used to allow injection of stimulation chemicals into specific zones in a multi-zone well completion. The device comprises a specialty shaped flapper valve that is normally closed across the inside diameter of the production string and that is opened by inserting a tubular member into the interior of the production string. The flapper valve element is shaped so as to closely conform to the shape of the inside diameter of the production string when the valve is opened, thereby allowing tools to be freely inserted into and withdrawn from the production string.
U.S. Pat. No. 5,139,981 (Le, 1992) describes a flapper-type check valve for controlling flow in a production string. The valve allows downward flow but blocks upward flow when closed and allows flow in both directions when open. The valve is opened by inserting a tubular member through the production string, thereby swinging the hinged flapper element downward to the inside surface of the housing. The key feature of this invention is a coil spring biasing assembly that is designed to provide a greater closing force to the valve element compared similar flapper valve devices.
U.S. Pat. No. 568,362 (MacSpadden et al., 1896) describes a check valve for pneumatic tires, footballs, and other inflatable devices. The invention comprises a flexible valve element and a flexible valve seat, both preferably comprised of sheet rubber.
U.S. Patent Application No. 2013/0312851 (Anderson et al., 2013) describes several embodiments of a low-profile valve that may be used as either a pressure relief valve or a check valve. Like the present invention, some embodiments of the Anderson invention are designed to be used in a well production string as a pressure-relief valve for venting excess fluid pressure from the interior of the production tubing to the annular space between the production tubing and the well casing. Like the present invention, the valve element and valve seat of the Anderson invention may be comprised of steel. In one of Anderson's embodiments, biasing force is supplied by the “stiffness of a rigid arm,” with one end of the arm attached to the valve element and the other end of the arm attached to the housing of the device in a cantilever configuration. This arm or spring configuration is different from the both-ends-fixed configuration of the biasing spring of the present invention.
In the Anderson invention, the components are designed and assembled so as to provide a minimum outside diameter of the device, thereby optimizing the device for use in applications in which the clearance between the production string and the veil casing is very limited. The disadvantage of this type of design is that the housing walls of the device are required to be relatively thin compared to other components in the production string, thereby reducing the overall tensile and compressive strength of the string. In contrast, in the present invention, the housing walls are designed to be relatively thick and strong, while the device is designed to fit in the annular space of wells onto typical sizes of well casings and production strings.
A first object of the present invention is to provide a relief valve that may be installed in-line in a string of well production tubing, providing for the release of excess pressure that may occur in the string due to a blockage, a closed control valve or other obstruction in the string, thereby preventing damage to the tubing or other components from overpressure. A second object of the present invention is to have a sufficiently small outside diameter so that a production string incorporating the present invention can easily fit within the annular space of a well casing that is matched to the size of the production string. A third object of the present invention is to have a sufficiently large inside diameter so that tools and pump rods that are designed to slip through the interior of a particular size production string will easily slip though the interior of the present invention when installed in the production string.